Gabriela is 4 times as old as William and is also 21 years older than William. How old is Gabriela?
Answer: We can use the given information to write down two equations that describe the ages of Gabriela and William. Let Gabriela's current age be $g$ and William's current age be $w$ $g = 4w$ $g = w + 21$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $g$ is to solve the second equation for $w$ and substitute that value into the first equation. Solving our second equation for $w$ , we get: $w = g - 21$ . Substituting this into our first equation, we get the equation: $g = 4$ $(g - 21)$ which combines the information about $g$ from both of our original equations. Simplifying the right side of this equation, we get: $g = 4g - 84$ Solving for $g$ , we get: $3 g = 84$ $g = 28$.